Several techniques are commonly used to communicate electrical signals from a transmitter to a receiver. Differential signaling is often used for such communications, where two complementary signals are sent on two separate wires. Universal Serial Bus (USB) is a common technique/protocol for communicating electrical signals using differential signaling.
A problem that is encountered with differential signaling is overshooting and undershooting of the differentially varying signals, which in turn cause electrical overstress (EOS) on devices that receive the signals. As an illustration, for a chip-to-chip USB serial communication implementation, a transmitter (TX) chip transmits data to a receiver (RX) chip located at some distant place on a circuit board. The on-board connection or communication link(s) between the transmitter and the receiver is through a USB connector, short conductor traces (typically made of copper), a “choke” circuit, long conductor traces, and pads that receive the two differentially varying signals. The rail-to-rail fast rising edges of the two differentially varying signals produce lot of electromagnetic interference (EMI) for nearby circuits or nearby electronic devices. Practically, the common mode of these differentially varying signals is not exactly at (VOH+VOL)/2, which can therefore lead to higher common mode noise as a source of EMI.
Since the EMI level is often mandated by government regulatory bodies to be below a certain level, the choke circuit is used in the communication link. The choke circuit typically contains mutually coupled inductors, and a feature of the choke circuit is to couple the two differentially varying signals tightly so that their common mode does not vary much and the common mode noise (in the form of EMI) is suppressed. Since the choke circuit is useful in such communication implementations, the choke circuit is often included in the communication link, and itself becomes a source of overshoot and undershoot if one of the signals makes a transition (such as from a high level to a low level, or vice versa) and the other signal does not make a transition. The undershoot is characterized by a downward spike or other significant dip in one or both of the signals, to a lower level of voltage than that produced elsewhere during transmission. The overshoot is characterized by an upward spike or other significant increase in one or both of the signals, to a higher level of voltage than that produced elsewhere during transmission.
This single-edge transition, which generates the overshoot or undershoot, may be observed for example in a single-ended zero (SE0) condition of the USB protocol at an end of packet (EOP) transmission. Generally, as long as the two signals are differential and edges of both signals make a transition simultaneously, overshoot and undershoot during transitions is less likely, due to the mutual coupling provided by the choke circuit. However, in the presence of a single edge (such as at an end of a packet EOP), the inductor(s) in the choke circuit generates kick back and the equivalent RLC tank circuit resonates, thereby giving rise to an under-damped response (e.g., ringing on both signals). This ringing results in severe overshoot and undershoot that can propagate through the long traces and then hit the receiver. The undershoot and overshoot can also affect a tristated transmitter at the receiver end.
Devices (such as drivers, transistors, and other circuitry) at the receiver-end that are connected to one or both pads in the communication link are exposed to the overshoot and undershoot, thereby raising EOS issues. For instance, the devices under EOS may not be designed to handle the voltage levels of the undershoot/overshoot, and so may degrade over time, and the reliability of the individual or overall circuitry is reduced.